Conventionally, display devices with a photodetection function capable of detecting ambient brightness by including a photodetecting element, such as a photodiode, in a peripheral region of pixels have been proposed. It is envisioned that such display devices with the photodetection function will be utilized as display devices capable of adjusting the light amount of a backlight in response to ambient brightness.
In some of these conventional display devices with the photodetection function, the photodiode is formed together in a semiconductor process for forming known components such as signal lines, scanning lines, TFTs (Thin Film Transistors) and pixel electrodes on the active matrix substrate (see Documents 1 and 3 below).
One of the major problems for the above-mentioned optical sensor at least some part of which is formed monolithically on an active matrix substrate of a display device is to eliminate the influence of light (stray light) other than the light to be primarily detected. In a display device having a backlight in particular, when output light from the backlight is reflected by a variety of components in the display device and the reflected light enters the photodiode as stray light, a significant error will be included in an optical sensor output.
Thus, in order to offset the influence of stray light, it has been known to provide, in addition to a detection photodiode, a photodiode (reference photodiode) that is configured not to receive light from the backlight and stray light (see Documents 2 and 4 below).
In conventional optical sensor including such a reference photodiode, generally, an offset to an output of the detection photodiode is corrected by inputting outputs of the detection photodiode and the reference photodiode to a differential amplifier and determining the difference between the outputs (see Document 4).
[Document 1] JP 2006-3857 A
[Document 2] EP 1394859 A2
[Document 3] T. Nakamura et al., “A Touch Panel Function Integrated LCD Including LIPS A/D Converter”, SID 05 DIGEST, pp. 1054-1055, 2005
[Document 4]S. Koide et al., “LIPS Ambient Light Sensor with Temperature Compensation”, IDW '06, pp. 689-690, 2006